With
practice, most birders learn to identify many species by
their characteristic calls and songs. What are the
underlying mechanisms that lead to "standardized"
repertoires for each species? Are these characteristic
sounds learned or are they genetically determined and fixed
without learning (innate)? Where learning is involved, when
does it take place? And how does each species "know" which
sounds are appropriate and should be learned? The answers to
these questions are as varied as birds themselves and have
long served as a focus of research by ornithologists,
ethologists, and neurobiologists.

Most of this research has
been concerned with song development in species of
songbirds, but relatively few species have been examined in
detail. Most songbirds must learn at least part of their
song repertoire. What little we know of vocal development in
nonpasserines indicates that calls of those species
(Mallard, American Coot, Ring-billed and Franklin's Gulls,
domestic chicken, and Ringed Turtledove) are innate rather
than learned, and that precocial young tend to have larger,
better-developed repertoires of calls than do altricial
young. Two exceptions among nonpasserines are hummingbirds
and parrots, for which there is some evidence of vocal
learning (there also is suggestive evidence for Greater
Prairie-Chicken and Sharp-tailed Grouse). Studies of many
groups have yet to be done.

The learning of songs is a
gradual process that takes place over a period of weeks or
months. Typically, a vague, jumbled "subsong" appears first
which then gradually is transformed into a more structured,
but still quite variable, "plastic song." The end point of
this process is the production of a stable repertoire of
"crystallized" songs. Much more material may be developed
than is actually needed for the eventual crystallized
repertoire, leading to a process of attrition as the mature
song takes shape. Swamp Sparrows, for example, generate four
to five times more song material during development than
they eventually retain in the adult repertoire.

The most thorough studies of
song development, pioneered by ethologists W H. Thorpe and
Peter Marler, involve detailed experimental procedures using
birds that have been isolated in soundproof chambers as
hatchings or nestlings. The development of songs and calls
can then be followed in these birds, which have been
deprived of any chance to hear the normal song of their
species. In most species that have been examined, the
resulting songs bear only a slight resemblance to normal
songs, and are not recognized by others of the same species.
Isolated birds allowed to hear the singing attempts of other
isolates form better, but still imperfect, songs, whereas
isolates allowed to hear normal song during their "sensitive
period" (the limited period of time during which song
learning can take place) develop normal songs.

The social bonds to the song
tutor (usually the male parent) have been shown to be
important in determining which vocalizations are copied by
young birds. In addition, territorial males appear to also
copy song characteristics of surrounding territorial males,
indicating that males of some species may have the ability
to expand their repertoires and replace song components each
breeding season.

Studies of song learning
have led to the "auditory template hypothesis" -- the idea
that each species is born with a neurological model of what
its song should sound like and it develops that song by
matching sounds that it hears with the template in its
brain. This process enables a young bird to filter out
inappropriate sounds and to produce sounds matching the
template, which are then stored for future use when breeding
the following spring. Swamp Sparrows are able to store and
precisely reproduce song syllables without rehearsal after
as long as nine months.

The template model is
exemplified by studies of young White-crowned Sparrows,
which manifest a sensitive period for song acquisition
roughly from day 10 through day 50 after hatching. Around
day 150 they begin to practice what they have learned, and
by day 200 they have developed a stable "crystallized" song
that matches parts of the song heard during their sensitive
period. Song learning is selective, so that if offered a
choice, birds will learn their own species' song. If offered
only songs of other species or if reared in isolation,
learning does not occur and only a simplified approximation
to the normal song develops. The importance of auditory
feedback is shown by birds that have been experimentally
deafened after exposure to normal song during the sensitive
period but prior to day 150. Such birds fail to develop
anything melodic. In contrast, birds deafened after their
own song had crystallized will continue to sing normally. In
essence, vocal learning appears to consist of two phases:
(1) exposure to and memorization of species-specific sounds
by matching them to the template during a sensitive period,
and (2) production of those sounds.

The duration and timing of
the sensitive period varies among species. Bewick's Wrens,
Song and Swamp Sparrows, and meadowlarks acquire their songs
during the first few months following hatching. In contrast,
Northern Mockingbirds, Indigo Buntings, and Red-winged
Blackbirds continue to incorporate new songs and song
elements into their repertoires beyond their first breeding
season. As additional field studies are conducted on
individually identifiable birds over several seasons, many
more species may be added to the latter category.